Where to start? Vertical Mill CNC conversion.

[Richard]

My first and second attempts at converting my mill seemed somehow wanting because I always wanted MORE!
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Eventually I figured out that I wanted zero backlash on all 3 axes, no power supplies to go wrong, (I had problems with power supplies), my own setup to locate the tool and work piece. When finished it was fricking incredible, but then I moved house, it got moth balled and I really miss it.
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What I lacked was a clear idea of what I wanted the mill to do. That could be a good starting point

To be honest, as long as it still works, I'll be happy! I don't want to destroy it.

I'm not too worried about the drivers side of things, more the mechanics of fitting the ballscrews.